The present invention relates to a system for insulating a cryogenic tank, such as a tank for space launchers or any other tank capable of containing a cryogenic fluid, such as notably a fixed storage tank, a tanker truck or a vehicle tank.
The use of cryogenic fluids for launchers of a space vehicle has particular interest since it makes it possible to employ liquid combustibles and fuels, notably for launching and maneuvering these machines. Moreover, these fluids are the subject of many developments for the propulsion of land vehicles.
Fluids capable of being contained in such tanks are, for example, hydrogen, oxygen, natural gas and ethylene. These fluids are stored in the liquid state at a very low temperature, for example hydrogen at below −252° C., oxygen at below −183° C., and natural gas at below −162° C., while the ambient temperature outside the tank is generally around 20° C.
The quantity of fluids necessary, notably for the propulsion of spacecraft, generally determines the form of tanks so that they are carefully adapted to the craft and these tanks constitute a very important part of the whole.
In order to maintain the fluid contained in the tank in the liquid state, the walls of the tank have to be insulated. Insulating materials are thus positioned between the structure of the tank, generally the outer wall of the tank, and the volume where the cryogenic fluid is stored.
Such a tank is for example described in U.S. Pat. No. 3,935,957 where the thermal insulating material is positioned between two walls.
An insulating material used in a known manner consists of a rigid cellular foam with a high proportion of closed cells, for example polyvinylchloride (PVC) or polyurethane (PU) notably with densities of which the order of magnitude is situated between 30 kg/m3 and 80 kg/m3. Reference is made by way of examples to the following commercial products: Airex® C71.55, Diab Divynicell® HP50, Puren® P 60 T.
It is usually considered that these materials constitute the best compromise in the terms of mass necessary for insulation, thermal performance, and ease of application, notably by adhesive bonding to the structure of the tank.
It should also be noted that the insulation, positioned inside or outside the tank, should be impervious in order to prevent cryogenic pumping of gas into the volume of the insulated material. To date, many materials have not been employed for thermal insulation of a cryogenic tank since they are not impervious.
It will be noted moreover that insulating materials used should have sufficient mechanical strength so as not to crack under stress fields induced by differential expansion between the structure of the tank and the insulating material, during filling or discharging cycles of the tank. Many materials crack or crumble when they are subjected to such temperature cycles and to the resulting high thermal gradients.
The result is that at the present time only a very limited number of insulating materials is employed for insulating cryogenic tanks.
U.S. Pat. No. 4,116,150 describes an insulating system according to the preamble of claims 1 and 13.